The present invention relates generally to methods for testing damage levels of semiconductor devices and more particularly to methods for measuring average power levels required for producing failures in p-n junctions of transistors. Various methods have been used in the prior art to predict the failure performance of semiconductor devices. Among those methods, the Wunsch test (Determination Of Threshold Failure Levels of Semiconductor Diodes And Transistors Due To Pulse Voltages, D. C. Wunsch and R. R. Bell, IEEE Transactions on Nuclear Science, Volume NS-15, page 244, December, 1968; Semiconductor Device Damage In An EMP Environment, D. C. Wunsch, Conference Proceedings, Component Degradation From Transient Inputs, Electromagnetic Effects Laboratory, U.S. Army Mobility Equipment Research and Development Center, 28-29 April 1970) has been the most successful for measuring the pulsed power failure thresholds for transistors. This information is essential to the selection of a semiconductor device to protect a circuit from extremely high power transient electromagnetic pulses (EMP) such as those produced by a nuclear blast. The well known method of protecting the circuit is to place a diode between the antenna of, for example, a receiver circuit, and ground potential in a reversed biased configuration. Incoming pulses which exceed the reverse bias breakdown voltage are therefore shunted to ground potential thereby protecting the elements of the circuit from burnout. However, to insure that the diode does not become shorted out itself by a pulse that exceeds its power handling capabilities and thereby shunting all incoming signals to ground potential so that the receiver is inoperable after the EMP has passed, a method is needed to nondestructively determine the power handling capability of each diode to be used. Tests to determine power handling capabilities of diodes are also applicable to p-n junctions of other semiconductor devices, such as transistors to predict their ability to handle high power EMP's within the circuit itself or as actual protection devices.
The Wunsch method is an attempt to measure the pulsed power failure threshold of both bipolar transistors and diodes. Wunsch utilizes a constant power pulse of reverse polarity and very short duration i.e., 100 nano-seconds or less, across a p-n junction of a semiconductor device. From the oscilloscope traces of voltage and current, the power absorbed by the p-n junction can be determined. By pulsing the device until failure occurs, the total power required to cause failure can be determined. If the area of p-n junction can then be determined, the power per unit area can be calculated so that the power capabilities of semiconductor devices which have not been subjected to tests can be calculated by knowing their p-n junction area.
However, the results of the Wunsch test do not have significant correlation to the expected theoretical results. According to theory, the maximum power per unit area before failure (P/A) is a function of both the Wunsch damage factor K and time t. In particular, EQU P/A = Kt.sup.- 1/2
The Wunsch damage factor K can be expressed as follows: EQU K= .sqroot.II C.sub.p k.pi. (T.sub.m -T.sub.i)
where
C.sub.p = specific heat for semiconductor material (i.e. silicon) PA1 k = Thermal conductivity for semiconductor material (i.e. silicon) PA1 .rho. = density of silicon PA1 T.sub.max = temperature at the p-n junction when burnout begins PA1 T.sub.i = Initial temperature of the p-n junction.
The measured results, however, vary significantly from the theoretical results which is due to a phenomenon referred to as second breakdown a condition differing from the primary breakdown (i.e., avalanche breakdown) -- the sustaining condition of a semiconductor device -- in that second breakdown is an irreversible failure condition. During second breakdown, the p-n junction does not act as a single unit to dissipate power. Normally, when a junction is in primary breakdown or avalanche condition there is an avalanche of charge carriers across the junction. During second breakdown, small sections (hot spots or current constriction points) of the p-n junction switch from an avalanche generating state to a thermal generating state and thereafter carry the bulk of the current across the junction. Therefore, power measurements made after second breakdown has occurred are useless in determining the power capabilities of the p-n junction since the junction has ceased to act as an integrated power dissipating unit because it has been at least partially damaged by the creation of one or more current constriction sites.
Studies by Sunshine and Lampert (Second-Breakdown Phenomena In Avalanching Silicon-On-Sapphire Diodes, R. A. Sunshine and M. A. Lampert, IEEE Transactions On Electron Devices, Volume ED-19, page 873, July, 1972) have confirmed the above theories pertaining to second breakdown. Sunshine and Lampert have studied second breakdown in silicon-on-sapphire p-n junctions under steady-state and pulse conditions. Their experimental techniques made it possible to observe local temperature rises in the interior of their diodes. By means of stroboscopic photographs of the temporal and spatial development of heated regions in operating diodes, they demonstrated the following facts about heat flow and the redistribution of current flow when their diodes were pulsed into second breakdown. During the delay time from the beginning of a square pulse of current to the time that second breakdown began, the avalanche current was more or less uniformly distributed across the junction plane and the junction space charge region heated rather uniformly. Second breakdown occurred when the thermally generated leakage current became large enough at some localized region of the junction to quench the avalanche there. The current then rapidly constricted and flowed through the hot spot. The power density at the constriction site was much higher than it had been during the delay time, and under some conditions a narrow, highly conducting hot filament propagated from the hot spot into the more resistive side of the diode. The formation of the hot filament usually resulted in device degradation and often in catastrophic failure. The filament formed and penetrated completely through the resistive region only when the junction temperature at the constriction site was much higher than the intrinsic temperature of the resistive material. When the constriction temperature was less than the intrinsic temperature, the hot region remained localized at the junction.
In the light of the Sunshine-Lampert studies, second breakdown is clearly a necessary anterior condition for device damage, since it indicates that reverse current in a p-n junction has switched from an avalanche to a thermal generating mechanism. Whenever second breakdown is observed, the power level and delay time preceding the breakdown are valid indicators of the failure threshold. Although the voltage drop occurs at second breakdown and can be readily seen in long-pulse testing (pulse duration greater than 100 nanoseconds), such is not always the case in short-pulse testing. In some short-pulse test cases the current apparently does not have time to constrict into a single channel, and the diode voltage declines gradually, if at all. In other cases, the second breakdown occurs just after the application of the test pulse but the fast rise time of the turn-on spike (caused by the inductance of the electrical connectors and circuit component leads) obscures the second breakdown voltage drop. The input power level computed from the remainder of such a voltage waveform is much lower than the true threshold failure level, and such data should be discarded.
The Wunsch test has added further to the miscalculation of data by use of a constant power pulse. When second breakdown occurs the resistance of the device falls thereby normally causing the voltage across the junction to fall to a lower level. To keep the power constant, it is necessary to increase the power across the junction to keep the voltage constant when second breakdown occurs thereby, in most cases, causing excessive, permanent damage to the device.
The total area under the constant power pulse curve is therefore indicative of the energy required to damage the device. The problem, however, is that the damage done to the semiconductor device under the Wunsch method is invariably catastrophic whether the applied power level was just at the threshold or considerably above it, and as a result, the quantum of applied power to cause threshold damage cannot be measured from the Wunsch method curves.